A digital to time converter (DTC) is an electrical circuit that translates a digital bit stream into an outgoing waveform. Typical DTCs, for example, convert a digital bit stream of logical “1”s and “0”s into an outgoing waveform such that the frequency and/or phase of the waveform varies in time to mirror the bit-pattern of the incoming digital bit stream.
A building block of a DTC is the phase multiplexer. The phase multiplexer is used to select dynamically between input phases (e.g. produced by phase locked loops “PLLs”) according to the modulation and without introducing glitches. Moreover, the phase multiplexer must be able to unwrap the phase (producing an output waveform that may have a delay larger than 1 clock period compared to the input unmodulated clock). The unwrapping is necessary to mimic the PLL behavior as a frequency modulator, and to ensure low out of band noise in the transmit output spectrum. The phase multiplexer achieves phase unwrapping by being able to cyclically select phases based within a period of the input clock.
Conventional phase multiplexers have relatively complicated architectures. Such complicated architectures are required because conventional phase multiplexers generally must change the selection for the phase twice every local oscillator period (e.g., 200 ps @2.5 GHz, or even a shorter time in presence of phase modulation). Furthermore, the architectures are complicated because it is known that conventional phase multiplexers create duty cycle modulation that requires correction by otherwise superfluous circuitry or functionality.